The understanding of the mechanisms involved in the sorting out and specific delivery of proteins from their site of synthesis to their site of function in the cell constitutes a central question in Cell Biology. One aspect of this problem is the surface polarity of epithelial cells. The well known ability of epithelial membranes, such as the lining of the intestinal tract and kidney tubular structures, to actively transport nutrients and electrolytes against steep concentration gradients has its origin in the asymmetric distribution of enzymes and transporting systems between the free, apical, region and the adherent, basolateral, region of the epithelial cell surface. A similar sort of surface asymmetry is responsible for the polarized exocytosis of exocrine secretory cells. The general objective of this proposal is to study the cellular mechanisms and the sorting out signals in membrane proteins responsible for this surface polarization using as a model system monolayers of polarized epithelial cells (such as a dog kidney cell line, MDCK) infected with enveloped RNA viruses (influenza, vesicular stomatitis, sendai, simian virus 5) which bud asymmetrically from the cell surface. We propose: To determine whether the segregation of apical and basolateral proteins is carried out intracellularly or after their insertion in the plasma membrane following the intracellular pathway and the site of surface insertion of viral envelope glycoproteins by immunoflourescence and ultrathin frozen section immunoelectronmicroscopy. To search for physiological and pharmacological conditions (calmodulin inhibitors, ionophores, prostaglandins, transglutaminase inhibitors, cytoskeleton disrupting drugs, reversion of transmonolayer electric gradient) that may specifically affect the intracellular migration of apical or basolateral proteins. To select mutants of MDCK cells defective in the surface expression of apical or basolateral proteins by immune complement cytolysis with monospecific antibodies. To search for mutants of influenza and VSV with opposite budding polarity. To produce a family of monoclonal antibodies against the Golgi apparatus in order to recognize subcompartments of this organelle possibly related to the sorting process. These studies should help to elucidate the molecular basis of polarized epithelial function. In addition, since epithelia are usually the first barriers to viruses, they may provide insights on how viral disease spreads in the organism.